Synaptosomal plasma membranes (SPM) were prepared from the cerebral cortex of four groups of rats: (i) controls; (ii) acutely treated with ethanol; (iii) dependent-intoxicated; and (iv) undergoing overt ethanol withdrawal syndrome. Estimates of the apparent microviscosity of these membranes over a range of temperatures indicated that temperature induced changes in SPM fluidity were smaller in ethanol-dependent rats. The cholesterol/phospholipid molar ratio was significantly increased in the SPM from intoxicated-dependent rats, and to a lesser extent in rats undergoing the withdrawal syndrome. The fatty acid content of SPM phospholipids was not significantly changed in any of the treatment groups. The in vitro addition of cholesterol to control membranes altered the apparent microviscosity in a manner similar to the changes found in SPM of intoxicated-dependent rats. These studies suggest that physical dependence upon ethanol may be related to changes in synaptosomal membrane composition and viscosity.